Re: [RFC PATCH 1/2] Fix: sched/membarrier: p->mm->membarrier_state racy load

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Sep 04, 2019 at 12:53:49PM +0200, Oleg Nesterov wrote:
> On 09/03, Mathieu Desnoyers wrote:
> >
> > @@ -1130,6 +1130,10 @@ struct task_struct {
> >  	unsigned long			numa_pages_migrated;
> >  #endif /* CONFIG_NUMA_BALANCING */
> >
> > +#ifdef CONFIG_MEMBARRIER
> > +	atomic_t membarrier_state;
> > +#endif
> 
> ...
> 
> > +static inline void membarrier_prepare_task_switch(struct task_struct *t)
> > +{
> > +	if (!t->mm)
> > +		return;
> > +	atomic_set(&t->membarrier_state,
> > +		   atomic_read(&t->mm->membarrier_state));
> > +}
> 
> Why not
> 
> 	rq->membarrier_state = next->mm ? t->mm->membarrier_state : 0;
> 
> and
> 
> 	if (cpu_rq(cpu)->membarrier_state & MEMBARRIER_STATE_GLOBAL_EXPEDITED) {
> 		...
> 	}
> 
> in membarrier_global_expedited() ? (I removed atomic_ to simplify)
> 
> IOW, why this new member has to live in task_struct, not in rq?

It could be like the above; but then we're still reading
mm->membarrier_state in the scheduler path.

The patch I just send avoids event that, at the cost of doing a
do_each_thread/while_each_thread iteration in the uncommon case where we
register a process after it grows threads.